During the switching of, in particular, direct currents with high current intensity and voltages over about 30 volts, the risk latently exists that during the opening of a contact gap constructed with switching contacts of a switch, an arc forms between the switching contacts via which a current flow is maintained. Such an arc destroys, on the one hand, the switching contacts of the corresponding switch and, on the other hand, involves the risk of a fire.
In order to prevent the risk of fire, special direct-current relays or contactors are known in which an arc that has occurred is conducted away from the switching contacts into special arc extinction chambers. To deflect the arc, magnets interacting with ionized particles of the arc or a stream of compressed air are used in this arrangement. These solutions are mechanically elaborate, on the one hand, and, on the other hand, unsuitable in installations in which no supply of compressed air is available, for example, in photovoltaic power plants.
Furthermore, solutions are known in which an arc is avoided, in that the current is diverted to a current bypass path in parallel to the switching contacts of the switch during the opening of the switch. This current bypass path can be formed by using semiconductor switches, capacitors or temperature-dependent resistors. In this arrangement, capacitors or temperature-dependent resistors in the current bypass path are designed in such a manner or semiconductor switches in the current bypass path are driven in such a manner that when the switch is opened, the current initially flows at least largely via the current bypass path so that voltage and currents are so low when the switch is operated that no arc occurs. After that, the current flow in the current bypass path is reduced, ideally to the value of zero, so that a current flow through the entire arrangement is interrupted.
Such an arrangement using a resistor having a positive temperature coefficient (PTC), also called a PTC resistor in abbreviation in the text which follows, is, for example, known from the printed document EP 0850486 B1. In order to be able to lower the current in the circuit to the value of zero after opening the switch and diversion to the current bypass path formed by the PTC resistor, this arrangement is operated in series with a further switch. The further switch can be opened without problems when the current through the bypass path has dropped to such a small value that, when the further contact gap is operated, an arc no longer occurs or would be extinguished. Both switches are operated simultaneously or at short time intervals from one another, for example by this being a jointly electromagnetically operated switch.
A similar arrangement is known from printed document DE 10 2005 006 953 A1. In a circuit suitable for DC or AC currents, two switches connected in series are provided, wherein a PTC resistor is connected in parallel with a first one of the switches. For interrupting the current in the DC- or AC-circuit arrangement, the first switch with the parallel PTC resistor is opened first. As a result, the current commutates to the current bypass path and flows through the PTC resistor without a switching arc occurring at the contacts of the first switch. Subsequently the PTC resistor heats up, as a result of which its resistance increases and the current in the DC- or AC circuit arrangement decreases. Afterwards, the second switch can as well be opened while the current is already decreased, in order to interrupt the current flow completely without the danger of an arc occurring at this second switch. The PTC resistor and a switching means for the second switch are coupled in such a way, that the second switch is automatically opened, when the PTC resistor expands according to its temperature rise.
However, it is disadvantageous in the arrangements described above that the PTC resistor in the current bypass path is regularly exposed to high loading with each switching process, which is why the arrangement is not suitable for a high number of switching cycles.
From printed document U.S. Pat. No. 4,583,146 it is known to provide a PTC resistor in parallel with a switching gap of a switch and further to provide a voltage dependent resistor (VDR) in parallel with the PTC resistor. After opening the switch and an according increase of the resistance of the PTC resistor, the voltage drop across the switching gap increases and the VDR resistor becomes increasingly conductive, thereby taking over a part of the current flow and unloading the PTC resistor. However, even then there is a possibility that the PTC resistor is not functional anymore after one or more switching cycles and is not able to suppress an arc reliably in a subsequent switching cycle.
In the case of switching members which must be designed for a higher number of switching cycles, alternatively a use of semiconductor switches in the current bypass path has therefore proven to be more suitable in practice, in spite of higher costs. Nevertheless, there is a risk of faults even with such a unit, for example, when the semiconductor switch is defective or the switching contacts are stuck or welded together. For safety reasons, therefore, it is usually required to provide a further circuit arrangement by means of which the circuit can be interrupted, and arcs at this further circuit and/or the first circuit arrangement being suppressed (prevented and/or extinguished).
However, equipping this further circuit arrangement with a semiconductor switch contributes to a further increase in cost for the total arrangement. In addition, there is no simple possibility in such a redundant arrangement of switches bridged with semiconductor switches to detect a failure of only one of the two arrangement parts, since a defect of only one half of the redundant arrangement does not detectably reduce the operability. It is possible, therefore, that the arrangement constructed to be redundant but defective on half its side is continued to be operated although this now increases the risk of the occurrence of an unsuppressed arc.